Pulse combusted acoustic agglomeration apparatus and process

ABSTRACT

An improved apparatus and process for removal of particulates entrained in a gas stream are provided. The removal process employs a pulse combustor to provide an acoustic pressure wave to acoustically enhance bimodal agglomeration of particulates which may be collected and removed using a conventional separation apparatus. A particulate having a size different from the size of the particulate in the gas stream to be cleaned is introduced into the system to effectuate the bimodal process. The apparatus may be employed as a direct fired system for improved operation of gas-operated equipment such as a gas turbine, or may, alternatively, be employed as an add-on subsystem for combustion exhaust clean-up. Additionally, the added particulate may be a sorbent for effecting sorption of other contaminants such as sulfur. Various other particulates for contaminant removal may also be introduced into the system as exemplified by alkali-gettering agents.

FIELD OF THE INVENTION

The present invention relates to a pulsed combustion apparatus andprocess for acoustically agglomerating particulates produced by thecombustion of fuels so that the particulates may be removed from thecombustion effluent stream.

BACKGROUND OF THE INVENTION

A major concern with the utilization of certain fuels to directly fireconventional power generation systems is the particulates produced bycombustion of the fuels. These particulates remain in the combustion gasstream. Because the gas stream running such systems can adversely impacton the life of turbines and the like, the gas stream should besubstantially free of particulate matter. Although conventional devicessuch as cyclones may be used to remove some of the larger solidparticulate matter from combustion gas streams, these devices generallyfail to remove the smaller particulates from the streams. Similarproblems also exist in many gas streams in which the particulatesuspended matter originates from other than combustion.

Removal of solid particulate from a gas stream is most important incoal-fuel advanced power generation systems. Particularly, a directcoal-fired gas turbine which uses gas turbines coupled in series withadvanced coal combustors has the potential to achieve high thermodynamiccycle efficiencies. The nature of the coal-based fuels, however, hasprevented efficient operation and effectiveness of these directcoal-fired gas turbine systems. Conventional gas turbine systemsnormally employ clean, premium-grade petroleum distillates in thecombustion system. In contrast, coal-based fuels produce ash andchemical species such as sulfur and fuel-bound nitrogen not found inappreciable quantities in the petroleum-based fuels. This mineral matterin coal-based fuels potentially impairs gas turbine efficiency, reducesreliability, increases maintenance costs, and adversely impacts theenvironment. Degradation of the coal-fired gas turbine's airfoilefficiency also occurs through corrosion, deposition, and erosionbrought about by particulates and other materials in the gas stream.

Direct coal firing of combustion gas turbines requires means to reduceor eliminate erosion of the turbine blades due to the presence of flyash and other particulates in the gas stream created by the burning ofcoal. If such erosion is not reduced, the turbine blade's life spanbecomes very short, on the order of 100 hours, thus compromising theeconomic viability of directly coal firing combustion gas turbines.

Direct coal firing may also result in the release of alkali vapors andsulfur compounds in addition to particulate combustion products. Suchcontaminant emissions cause the turbine blades to corrode quickly.

Fuel-bound nitrogen causes nitrogen oxide (NO_(x)) emissions to alsoform in the gas stream. Although nitrogen oxides do not affect turbineblades per se, they do represent pollutants that are not desirable inthe atmosphere. Methods and processes to either reduce the production ofnitrogen oxides or to destroy or remove such pollutants from the fluegas stream are necessary to meet the requirements of the Clean Air Act.Economically viable means for removing pollutants from the turbineexhaust before discharging such exhaust into the atmosphere have notheretofore been available.

Various attempts have been made to overcome the above and other problemsto provide an economically feasible and efficient process for directsolid fuel firing of gas turbines. Attempts have also been made toprovide a method for removing fine particulates from a gas stream. Forexample, coal fuels have been ultracleaned prior to combustion to reducecoal-based contaminants. This, of course, imposes substantial financialburdens as well as delays in time for utilization of the coal. In onesuch approach, the coal is extensively cleaned in an attempt to removeash and sulfur from the fuel prior to firing. A cold water slurry ismade from micronized, deeply cleaned coal and then used as fuel. Thisapproach, of course, is expensive but does produce an essentiallyoil-like slurry fuel made from coal that requires little modification tothe gas turbine engine, The cost of the necessary coal cleaning andslurry preparation, however, is sufficiently high that this approach hasessentially been abandoned.

Other attempts to create clean combustion gas streams have utilizedmodestly clean fuel products in connection with a hot gas clean-upsystem upstream from the gas turbine. Most of the particulate controldevices are secondary or tertiary particulate control devices in thatmultiple clean-up stages are required to sufficiently clean theparticulate-laden gas stream. Generally, such approaches have used aslagging combustor concept for the removal of the bulk ash particulate.The gas turbine coal combustors operate at sufficiently high temperatureby controlling the stoichiometry of the combustion air to nearstoichiometric, in an adiabatic combustion chamber, so that ash becomesmolten and is removed in the form of slag from the flue gas. Thisapproach, however, retains significant amounts of residual fine ashparticles (with an average size of 4 microns) in the gas stream whichare sufficient to harm the turbine blades.

Slagging combustor systems also often utilize high temperature ceramicfilters downstream of the gas turbine combustor and upstream of theturbine itself to capture residual fine ash particulates before theyenter the turbine and erode or otherwise damage the turbine blades.Ceramic filters, however, admit a very low surface gas velocity, thuscausing a large and unacceptable pressure drop across the filter. Thiscauses the size of such ceramic filters to become prohibitively largeand, therefore, very expensive. Furthermore, ceramic filters areunreliable because they are extremely fragile and susceptible to thermalshock and the thermal stresses resulting therefrom. In addition, suchfilters tend to plug, thereby requiring means for keeping the filtersclean without causing a steady pressure drop across the filter as it"loads up" with fine particles.

The high temperatures at which the slagging combustors must operate alsotend to increase the amount of nitrogen oxides produced in thecombustion process. This, in turn, requires other means downstream fromthe coal combustor to reduce the concentration of nitrogen oxides in theeffluent gas stream.

The high combustion temperatures in the slagging combustors operate atan inappropriate temperature for sulfur capture using dry sulfursorbents such as limestone or dolomite. Sulfur oxides produced byburning the sulfur-laden coal fuels must, therefore, be removed from theflue gas stream somewhere downstream of the combustor. A further sideproduct created by the high temperature in a slagging combustor is therelease of alkali vapors in the gas stream that must also be removed toreduce corrosion of the turbine blades.

Other non-slagging designs utilize dry ash rejection upstream of theturbine. In these designs, sulfur is captured using dry sorbents in atri-stage combustor. A multi-stage modular design of a combustionapparatus in this approach utilizes a modified tri-stage combustormodified for ash rejection and sulfur capture. An aerodynamic particleseparator separates ash rejection. This system has been found to producehard black deposits on the surface of the combustor quench zone.Involuntary slagging in the quench zone thus results, with hardenedpieces breaking off and traveling downstream without resettling on othersurfaces of the combustor which could damage the system. Further gasclean-up and nitrous oxides controls must also be employed downstream ofthe combustor.

Other systems utilize fabric-filter technology to control emissions instandard boiler applications. Fabric filters, however, are notapplicable to the gas turbine hot gas clean-up systems.

In summary, effective reduction of suspended particulates in a gasstream created by combustion remains a paramount problem due to the lackof a cost effective, efficient system for particulate removal,particularly very small particulates. Available particulatecollection/removal systems are limited by generator operatingconditions. New innovative approaches are thus needed to provide asystem so that fuel which produces particulates may be employed tooperate generators requiring highly cleaned gases. Any such new systemshould possess a number of attributes, such as high combustionefficiency, high sulfur capture capability, high solid fuel particulateremoval, low nitrogen oxide emissions, and high removal of alkali vaporscreated by the combustion of the fuel. Moreover, a new system providingthe above attributes should also be relatively inexpensive and shouldnot require substantial preparation and pre-cleaning of the fuel usedfor combustion.

Acoustic agglomeration is a process in which high intensity sound isused to agglomerate submicron- and micron-sized particles in aerosols.Agglomeration is, in essence, a pretreatment process to increase thesize distribution of entrained or suspended particulates to enable highcollection/removal efficiencies using cyclone or other conventionalseparators. Acoustic waves cause enhanced relative motion between thesolid particles, and hence, increases collision frequency. Once theparticles collide, they are likely to stick together. As an overallresult of acoustic influence, the particle size distribution in theaerosol shifts significantly from small to larger sizes relativelyquickly. Larger particles may be more effectively filtered from thecarrying gas stream by conventional particulate removal devices such ascyclones. The combination of an acoustic agglomeration chamber with oneor more cyclones in series provides a promising high-efficiency systemto clean particulate-laden gases such as hot flue gases from pressurizedcombustors.

Acoustic agglomeration of small particles in hot combustion gases andother sources of fine dust-bearing effluent streams has been studiedintermittently for many years. Although effective in producinglarger-sized particles (5 to 20 microns) for more efficient removal byconventional devices, the prior art methods of acoustic agglomerationare not generally viewed as potential clean-up devices due to theirlarge power requirements. For example, fine fly ash particulates (lessthan 5 microns in size) have been agglomerated using high-intensityacoustic fields at high frequencies in the 1,000-4,000 Hz range. Thesehigher frequencies were necessary for the disentrainment of the fineparticulate so as to effect collisions therebetween, and hence,agglomeration of the fine particles.

In such prior art acoustic agglomeration devices, the acoustic fieldshave been produced by sirens, air horns, electromagnetic speakers, andthe like. The resulting acoustic wave generation for sonic agglomerationrequires power estimated to be in the range of 0.5 to 2 hp/1,000 cfm.Significant parasitic power loss is therefore present as notedhereinafter even for efficient horns, sirens and the like which normallyhave efficiencies ranging from 8 to 10%.

Sirens, air horns and the like require auxiliary compressors topressurize air needed to operate same. The pressure required isgenerally well above the pressure available at the gas turbinecompressor exit, thus necessitating a means for providing that requiredpressure if the turbine is to be employed, or utilization of anauxiliary compressor. Electromagnetic sonic devices require specialdesigns and precautions to provide the desired equipment reliability,availability and life. Likewise, powerful amplifiers are required todrive certain speakers in order to deliver 160 decibels (dB) or more ofsound pressure. All of the above acoustic systems are thus inefficientfrom at least a cost standpoint.

Apparatus and processes according to the present invention overcome theabove-noted problems of the prior art and possess the desired attributesset forth above by using a pulse combustor arrangement for acousticallyenhanced bimodal agglomeration of particulate in a gas stream.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an improvedapparatus for removing particulate from a gas stream.

Another object of the present invention is to provide an improvedcombustor that operates on high sulfur fuels such as coals whileproviding for clean-up of particulates produced by the burning of suchfuels and avoiding unwanted gaseous emissions.

Still another object according to the present invention is to provide ahigh efficiency pulse combustor system to acoustically-enhanceparticulate agglomeration.

Another object according to the present invention is to provide animproved process for removing particulate from a gas stream.

It is yet another object of the present invention to provide a means forremoving alkali vapors produced during a fuel combustion process.

Another object of the present invention is to provide for contaminantcapture and particulate agglomeration of products produced by combustionin a single pass of a gas stream.

Another object according to the present invention is to provide anapparatus for creating a low frequency acoustic field to enhanceagglomeration of particulates produced during combustion.

A further object according to the present invention is to provide apulse combusted contaminant removal subsystem for adding to the exhaustsystem of an existing combustor.

Another object according to the present invention is to provide a pulsecombusted apparatus for removing gas stream particulates that producesreduced nitrogen oxides emissions.

Another object according to the present invention is to provide a meansfor capturing and removing sulfur derivatives from a combustion gasstream.

It is yet another object of the present invention to provide an improvedprocess for removal of particulates from a gas stream.

Another object of the present invention is to provide an improvedprocess for sorption of contaminants from a gas stream and bimodalparticulate agglomeration in a single pass through the system.

Yet another object of the present invention is to provide a process toaccomplish enhanced cleaning of a gas stream.

Generally speaking, apparatus according to the present inventionincludes a means for receiving a gas stream so that a gas stream havingparticulates suspended therein may pass therethrough, a pulse combustormeans in communication with the gas stream receiving means, said pulsecombustor means being capable of producing a pulsating stream of hotcombustion products and an acoustic wave having a frequency within therange of from about 20 to about 1500 Hz which acts on the gas stream,and means for introducing a second particulate into said gas stream sothat acoustically enhanced bimodal particulate agglomeration occurs forsubsequent enhanced particulate removal.

More specifically, in preferred arrangements, the second or furtherparticulate is introduced into the combustion product stream within thepulse combustor means, and most preferably at or near the junction ofthe combustion chamber and the resonance tube(s). Likewise in preferredarrangements, the second particulate is a sorbent for a contaminant inone of the streams, such as a sulfur derivative which enables sorptionof the contaminant and bimodal agglomeration of the suspendedparticulate matter. In such an arrangement, introduction of the sorbentat or near the junction of the combustion chamber and the resonance tubeyields a highly porous sorbent for better sorption of contaminant.

Generally speaking, the process according to the present inventionincludes the steps of pulse combusting a fuel to produce a hotcombustion product stream and an acoustic pressure wave having afrequency within the range of from about 20 to about 1500 Hz to act on agas stream having particulates suspended therein and introducing intothe gas stream a second particulate having a size distribution differentthan the size distribution of the suspended particulate in the gasstream so that acoustically enhanced bimodal agglomeration ofparticulates occurs to permit enhanced removal.

More specifically, in the present process, the size distribution of theintroduced particulate is preferably greater than the size distributionof the suspended particulate, and is also a sorbent for contaminants inthe gas stream such as sulfur compounds. Further, in a preferredarrangement, the particulate, and especially if a sorbent, is introducedinto the pulse combustor means around the junction of the combustionchamber and resonance tube.

BRIEF DESCRIPTION OF THE FIGURES

The construction designed to carry out the invention will be hereinafterdescribed, together with other features thereof. The invention will bemore readily understood from a reading of the following specificationand by reference to the accompanying drawings forming a part thereof,wherein an example of the invention is shown and wherein:

FIG. 1 is a schematic illustration of an apparatus for removal ofparticulates entrained in a gas stream according to the teachings of thepresent invention.

FIG. 2 is a schematic illustration of a further embodiment an apparatusfor removal of particulates entrained in a gas stream according to thepresent invention.

FIG. 3 is a schematic illustration of an apparatus for removal ofparticulates entrained in a gas stream according to the presentinvention shown as a clean-up system added on to the exhaust system ofan existing combustor.

FIG. 4 is a schematic illustration of an apparatus for removal ofparticulates entrained in a gas stream shown as an add-on system to anexisting combustor-driven turbine.

FIG. 5 is an illustration of one preferred pulse combustor design.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One preferred apparatus for removing particulates entrained in a gasstream according to the present invention integrates a pulse combustormeans with a particulate collection/removal means as illustrated in FIG.1, prior to gas introduction to a turbine generator. In FIG. 1, a pulsecombustor means, generally 10, is connected in line in communicationwith particulate collection/removal means, generally 20, so thatagglomerates formed in a gas stream passing therethrough can be removedfrom the gas stream by collection/removal means 20. After theparticulate collection/removal means 20 separates particulates from thecombustion gas stream, the gas stream continues, in this particularembodiment, to operate a gas-operated turbine generator 40. Turbine 40generates rotary mechanical power supply to a generator 50 and an aircompressor 60. Because the gas stream supplied to turbine 40 has beensubjected to the acoustic agglomeration and removal of particulatematerial according to the present invention,.the gas stream issufficiently clean for operating turbine 40 without significant adverseimpact on turbine 40.

Pulse combustor means 10 includes a fuel valve means 12 which preferablyis an aerodynamic valve (fluidic diode), though a mechanical valve orthe like may also be employed. A combustion chamber 14 is incommunication with valve means 12 and receives fuel-air mixturestherefrom on demand. A resonance tube or tailpipe 16 is in communicationwith combustion chamber 14. The apparatus of the present inventionfurther includes means 15 for introducing second particulate into theagglomerator. This added particulate is preferably introduced at pulsecombustor means 10 as illustrated and will combine with particulates inthe hot combustion product stream to form agglomerates as discussedhereinafter. Additionally, pulse combustor means 10 may include an airplenum 18 and a thrust augmenter (not shown). Resonance tube 16 may be asingle tube or tailpipe as shown or a plurality of tubes and, in onepreferred arrangement, continuously flares outwardly from combustionchamber 14. Resonance tube 16 with a flared configuration acts as adiffuser to reduce the gas exit velocity from combustion chamber 14 andprovides for recirculation of combustion products and increasedparticulate resonance time within the pulse combustor means 10.

In the embodiment shown in FIG. 1, compressed air from compressor 60 isfed to air plenum 18 for auqmenting the thrust of the fuel mixture fedinto pulse combustor means 10, though such is not necessary. Theresonance chamber 16 is arranged so that its outer free end permitscombustion products produced in combustion chamber 14 to enter intomeans for receiving a gas stream exemplified by section 19, though asmentioned hereinafter, many different arrangements are within thepurview of the present invention. Gas flows through receiving means 19and agglomeration of particulate therein occurs as described herein.

In the particular embodiment shown in FIG. 1, the pulse combustorapparatus is a self-contained direct fired system as opposed to anadd-on emissions control subsystem described as shown in FIG. 3. Hence,the gas stream is the stream of combustion products from combustionchamber 14, and contains undesirable particulates to be cleanedaccording to the present invention and supplied to turbine 40.

The particulate collection/removal means 20 in communication with pulsecombustor means 10 may employ a cyclone 72, bag house, scrubber or otherconventional solids separator device. As shown in FIG. 1, cyclone 72 isprovided with a duct pot 74 having an opening 76 for removal of solidwaste therefrom. Collection/removal means 20 is also in communicationwith gas turbine 40 so that the cleaned gas stream may act directlythereon in a proper operational mode. The entire apparatus may berefractory-lined or may be water-cooled, depending on the heatrequirements of the system.

In the embodiment shown in FIG. 1, the air plenum 18 communicates withbypass air conduits 17 through which additional air may enter into thegas stream receiving means 19 so as to further increase agglomeration ofthe particulates.

A pulse combustor such as preferably employed in the present inventiontypically includes at least one aerodynamic valve or fluidic diode, acombustion chamber and at least one resonance tube. An appropriate fueland air mixture passes through the valve into the combustion chamber andis detonated. During start-up, an auxiliary firing device is provided.Explosion of the fuel mixture causes a sudden increase in volume andevolution of combustion products which pressurizes the combustionchamber. As the hot gas expands, preferential flow in the direction ofthe resonance tube is achieved with significant momentum. A vacuum isthen created in the combustion chamber due to the inertia of the gaseswithin the resonance tube. Only a small fraction of exhaust gases arethen permitted to return to the combustion chamber, with the balance ofthe gas exiting the resonance tube. Because the pressure of thecombustion chamber is then below atmospheric pressure, further air-fuelmixture is drawn into the combustion chamber and auto-ignition takesplace. Again, the valve means thereafter constrains reverse flow, andthe cycle begins anew. Once the first cycle is initiated, operation isthereafter self-sustaining.

The fuel valve utilized in many pulse combustion systems is a mechanical"flapper valve" arrangement. The flapper valve is actually a check valvepermitting flow into combustion chamber, and constraining reverse flowby a mechanical seating arrangement. Although such a mechanical valvemay be used in conjunction with the present system, an aerodynamic valvewithout moving parts is preferred. With aerodynamic valves, during theexhaust stroke, a boundary layer builds in the valve and turbulenteddies choke off much of the reverse flow. Moreover, the exhaust gasesare of a much higher temperature than the inlet gases. Accordingly, theviscosity of the gas is much higher and the reverse resistance of theinlet diameter, in turn, is much higher than that for forward flowthrough the same opening. Such phenomena, along with the high inertia ofexhausting gases in the resonance tube, combine to yield preferentialand mean flow from inlet to exhaust. Thus, the preferred pulse combustoris a self-aspirating engine, drawing its own air and fuel into thecombustion chamber followed by auto-ignition.

Pulse combustor systems regulate their own stoichiometry within theirranges of firing without the need for extensive controls to regulate thefuel feed to combustion air mass flow rate ratio. As the fuel feed rateis increased, the strength of the pressure pulsations in the combustionchamber increases, which in turn increases the amount of air aspiratedby the aerodynamic valve, thus allowing the combustor to automaticallymaintain a substantially constant stoichiometry over its designed firingrange. The induced stoichiometry can be changed by modifying theaerodynamic valve fluidic diodicity.

The preferred pulse combustor used herein for coal-firing is based on aHelmholtz configuration with an aerodynamic valve. The pressurefluctuations, which are combustion-induced in the Helmholtzresonator-shaped combustor, coupled with the fluidic diodicity of theaerodynamic valve, causes a bias flow of air and combustion productsfrom the combustor's inlet to the resonance tube exit. This results inthe combustion air being self-aspirated by the combustor and for anaverage pressure boost to develop in the combustion chamber to expel theproducts of combustion at a high average flow velocity (over 1,000feet/second) through the resonance tube.

The production of an intense acoustic wave is an inherent characteristicof pulse combustion. Sound intensity adjacent to the wall of the pulsecombustion chamber is often in the range of 110-190 dB, and may bealtered depending on the desired acoustic field frequency to accommodatethe specific application undertaken by the pulse combustor.

Rapid pressure oscillation through the combustion chamber generates anintense oscillating flow field. In the case of coal combustion, thefluctuating flow field causes the products of combustion to be sweptaway from the reacting solid coal thus providing access to oxygen withlittle or no diffusion limitation. Secondly, pulse combustors experiencevery high mass transfer and heat transfer rates within the combustionzone. While these combustors tend to have very high heat release rates,(typically ten times those of conventional burners), the vigorous masstransfer and high heat transfer within the combustion region result in amore uniform temperature. Thus, peak temperatures attained are muchlower than in the case of conventional systems, and results in asignificant reduction in nitrogen oxides (NO_(x)) formation. The highheat release rates also result in a smaller required combustor size fora given firing rate and a reduction in the resonance time required.

The present invention is especially useful when the pulse combustor unitburns low cost, high sulfur, high ash, standard grind (pulverized) coalfuels. The particulate agglomeration and its efficient removal by thepresent invention permits use of standard grind coal in the presentcombustion device. This coal, in particular, yields increased ashparticulate size over that produced by combustion of micronized fuels,which provides more foci for the agglomeration of fine fly ashparticulate in the combustion gas stream at lower frequencies asdescribed herein. An economic advantage is clearly obtained becausestandard grind fuels are less expensive than micronized coals. It isalso advantageous to use coals that are not deeply beneficiated forextensive reduction of ash content. The increased mass loading of mediumto large ash particulate which would be produced from the combustion ofmoderately beneficiated fuels contributes to the efficiency of thebimodal dynamic filter effects in agglomerating particulates accordingto the present invention. Of course, use of the standard grind fuel alsoresults in a higher content of contaminants such as sulfur derivativesand in particular sulfur dioxide, and the release of alkali vapors, suchas sodium chloride, potassium chloride, and sodium sulfate. Thesefurther contaminants, however, may be effectively removed from the gasstream according to the present invention and combustion products formedfrom the standard grind coal may be effectively agglomerated andremoved.

The oscillating flow field created by the pulse combustor provides forhigh interphase and interparticle mass transfer rates. Due to thereasonably high temperature, combustion of fuel fines is substantiallycomplete at the exit of the pulse combustor resonance tube. Also,temperature may be maintained below that necessary for ash fusion for anon-slagging process as is preferred. However, temperature may beelevated to ash fusion temperature for a slagging process if desired.Further, additional residence time in the outwardly flaring resonancetube insures high carbon conversion and, in turn, high combustionefficiency.

Devolatilization and combustion of fuel fines in the pulse combustoralso enable the release of a significant portion of the sulfur in thefuel by the time the fuel fines leave the tailpipe or resonance chamber.According to the present invention as more particularly definedhereinafter, the introduced particulate can be, and preferably is, asorbent for sulfur which affords a high probability of sulfur capture bythe sorbent particulate. Fines recirculation as a consequence of thedesign of the resonance tube also assists in achieving high sulfurcapture efficiency at low Ca/S molar feed ratios, which leads to lowersorbent and waste disposal costs.

Pulse combustors are inherently low NO_(x) devices. The rate of heattransfer in the pulsating flow is higher than in the conventional steadyflow systems, resulting in a lower overall temperature in the combustionchamber. Also, the high rates of mixing between the hot combustionproducts and the colder residual products from the previous combustioncycle and the incoming cold reactants create a short resonance time athigh temperature thus quenching the NO_(x) production. Thesecomplementary mechanisms create an environment resulting in low NO_(x)production. Consequently, the NO_(x) emissions from systems of thepresent invention are believed to be lower than that of conventionalcombustors.

The direct-fired pulse combustion particulate removal system shown inFIG. 1 operates in the following manner. A fuel-air mixture is admittedto the air plenum 18 and then through one or more valve means 12 tocombustion chamber 14. The initial mixture entering the combustionchamber 14 is ignited by an ignition means such as a spark, a gas burneror the like 14'. The combustion products formed then resonate throughresonance tube 16. As described above, once the initial combustion cycleis begun, the pulse combustion becomes self-sustaining.

Pulse combustor means 10 produces an intense acoustic wave bycombustion-induced pressure oscillations when fired with a suitablefuel. The sonic field produced by combustion resonates through theresonance tube 16 and acts directly on the gaseous stream carryingparticulates. No compressed air (such as that which drives a siren or anair horn) or electricity (used to drive an electromagnetic speaker) isrequired. As explained above, however, additional compressed air (whichmay be recycled) may be supplied to the air plenum 18 for increasedthrust augmentation. The pulse combustor thus eliminates the need forparasitic power to generate the acoustic field.

In the direct coal-fired apparatus and process according to the presentinvention, the pulse combustor means produces a pulsating stream of hotcombustion products having a first particulate entrained therein. Thisfirst particulate is generally fly ash fines resulting from combustionof the fuel-air mixture and of a size of about 4 microns. The acousticwave produced by pulse combustor means 10 acts on the gaseous stream toeffectuate an acoustically-enhanced bimodal agglomeration of theparticulate in the gaseous stream when extra or second particulate of adifferent size distribution is present. Bimodal production ofparticulate agglomerates enlarges overall particulate size and therebypermits enhanced removal of the agglomerates by conventional means.Efficiency of the agglomeration process is enhanced by increasing thetotal mass loading of particulate dispersed in the gaseous stream. For agiven size distribution, a higher mass loading provides more particlesper unit volume and, consequently, an enhanced likelihood for collisionsamong the particles which leads to agglomeration. Accordingly, theaddition of a second mode, herein referred to as the introduction of asecond particulate into the hot combustion product stream or otherstream increases the total mass. Therefore, a more efficientagglomeration process is produced by the bimodal procedure.

In addition to this mass-dependent phenomenon, a further increase inparticle collisions, and therefore an increase in agglomeration, isproduced due to intensified orthokinetic interactions between the twomodes. Hydrodynamic interactions also occur. Preferably, the secondparticulate introduced into the hot combustion product stream will havea larger size distribution than particulate already entrained in the gasstream wherefore more relative motion occurs among the particles,enhancing agglomeration.

The second particulate material also is preferably introduced at or nearthe interface between the resonance tube 16 and the combustion chamber14 which is a region of high heat release and high heat transfer,particularly when the second particulate is a sorbent for sulfur or thelike. High heat thus provides a rapid rate of sorbent calcination,providing high porosity in the calcined sorbent which, in turn,generates high surface-to-mass ratios without the need for micronizationof the sorbent. Together with the effect of the oscillating flow fieldon gas mass transfer, introduction of particulate at a point at whichthe acoustic wave may act on the particulate thereby enhances sorbentutilization at relatively low calcium-to-sulfur molar ratios.

Pulse combustors according to the present invention produce lowfrequency acoustic fields having frequencies in the range of from about20 to about 1500 Hz. Higher frequencies give rise to more cycles ofparticulate matter per unit time within the gas stream acted on by theacoustic field. However, higher frequencies tend to yieldproportionately smaller amplitude of relative motion between theparticulates per cycle.

As previously explained, higher frequencies often effectuate significantdisentrainment of fine particulates in the gas stream. However, because,as explained herein, the particle size of the second particulateintroduced into the hot combustion product stream is preferably selectedto be larger than particulates already in the stream, the frequencyrequired to effect disentrainment is reduced. For example, a 100 micronparticulate in a gas stream at 1600° F. and 10 atmospheres would haveabout a 0.1 entrainment factor at a frequency of only 100 Hz. Thus, theoscillatory displacement amplitude of the particulate is only one-tenththat of the gas stream with significant (about 90% of gas displacementamplitude) relative displacement between the gas stream and theparticulate. The fine ash particulates, however, would be essentiallytotally entrained in the oscillating gas flow field with entrainmentfactors in excess of 0.99 at a frequency of about 100 Hz. This, in turn,leads to collisions between the fine ash particulates and the largersorbent agglomeration foci, causing agglomeration of the fine ashparticulates to the second particulates. Because the amplitude ofrelative motion between the fine ash particulates entrained in the gasstream and the second particulates would be on the order of 80-90% ofthe amplitude of the oscillatory gas displacement and such displacementsare higher for low frequencies, the collisions per cycle of oscillationsis thereby increased. Thus, this bimodal approach to particulateagglomeration, which basically means that particulates of a differentsize distribution are agglomerated together, provides a form of adynamic filter to collect the fine ash particulates on the second,larger particulates introduced into the gas stream.

Preferably, the pulse combustor will generate acoustic fields havingfrequencies of from about 50 to about 250 Hz. The high intensityacoustic fields also preferably have sound pressure levels of generallygreater than 160 dB, resulting in significantly higher mass transportproperties. These properties increase sorbent utilization by increasingthe rate of sulfur derivatives transported to the sorbent particlesurfaces and increases penetration into the pore structure of thecalcinated sorbent particles. The high intensity acoustic field furtherenhances acoustic agglomeration, changing the particulate sizedistribution such that small micron and submicron particulates areagglomerated to become larger particulates which can then be removedmore efficiently by conventional particulate removal devices.

According to the present invention, capture of contaminants such assulfur derivatives, occurs simultaneously with the agglomeration of theparticulates in the gas stream. The second particulate which isintroduced into the gas stream, particularly in a coal-fired operation,is preferably a sulfur sorbent such as limestone, dolomite, hydratedlime, or the like and is preferably selected so that its particle sizeis larger than the size of the particulate to be agglomerated. Secondparticulate size distribution is preferably in a range of from about 100to about 150 microns. Larger second particulate reduces the frequencyrequired to effect significant disentrainment of the particulate in thegaseous stream. This leads to collisions between the particulate and thelarger sorbent particles thus agglomerating the particulates to thesorbent. Particle size and size distribution as used herein shall referto sizes of particles within a distribution scheme. Hence, a larger sizeor size distribution of a particulate refers to a size distributionscale in which larger particles are present.

Porous calcium oxide sorbent particles react readily with the sulfurderivatives such as SO₂ contained in the hot combustion gases under theinfluence of the intense acoustic field to form solid calcium sulfate(CaSO₄) in and around the pores of the sorbent as well as on thesurface. The sulfated particles and particulate ash from the combustionprocess are agglomerated and may be easily removed from the gas stream.

Effective acoustic enhancement of particulate agglomeration will be mostpronounced as sorbent particle size increases. This is due to the factthat the acoustically-enhanced intraparticle flow will ameliorate theincreasing diffusional limitations encountered for larger particlesizes. Both enhanced desulfurization and efficient particleagglomeration and removal can be simultaneously achieved without thepenalty of using expensive sorbent materials. Simultaneous contaminantcapture and agglomeration as used herein refers to generally same timeand in the same pass.

Once the agglomerates are formed, the gas stream containing same travelsthrough collection/removal means 20 where the agglomerates aredisentrained from the gas stream and removed. The cleaned gas stream maythen be used to drive gas turbine 40 which in turn may power compressor60 and/or generator 50. If turbine 40 or some other device to beoperated by the gas requires a heated gas, then the collection/removalmeans may be heated to maintain the gas stream at a desired elevatedtemperature. The cleaned gas stream containing essentially gaseousproducts without significant pollutants such as sulfur derivatives andnitrogen oxides may be emitted to the atmosphere without significantpollutants.

A further embodiment of apparatus of the present invention is shown inFIG. 2, and includes for elements shown in FIG. 1, like numbered membersindicating like elements. An injection port 23 is included incommunication with combustion chamber 14 of pulse combustor 10 beyondvalve means 12 for further fuel addition. Also, instead of acontinuously flaring resonance chamber 16 as shown in FIG. 1, FIG. 2illustrates a relatively straight resonance chamber 16 having a diffusersection 21 at its outer end. Diffuser section 21 allows forrecirculation of fines to decrease NO_(x) emissions.

Like the apparatus of FIG. 1, FIG. 2 illustrates a second particulateintroduction means 15 for adding second particulate into the hotcombustion products stream. Second particulate introduction inlet 15 maybe located along resonance chamber 16 as shown in FIGS. 1 and 2 or may,alternatively, be located anywhere within the particulate removalapparatus where the particulate will be acted upon by the acoustic wavegenerated by the pulse combustor. For example, second particulate may beintroduced into the apparatus at a point beyond the resonance tube 16.In such an apparatus, the second particulate introduction means shouldbe positioned so that the acoustic wave will act on the gas stream forenhanced agglomeration between the particulates. As explained above, thesecond particulate is preferably a sorbent for effecting sorption ofvarious contaminants such as sulfur derivatives. Moreover, because theinvention relates to a bimodal apparatus and process, the sizedistribution for the second particulate introduced into the gaseousstream should be different from that of the particulate initiallycontained in the gas stream. In the most preferred embodiment, thesecond particulate should have the larger size.

Apparatus of FIG. 2 further employs a further inlet means 27 forintroducing a third material into the gas stream. Alkali vapors such assodium chloride, potassium chloride and sodium sulfate are often formedduring the combustion of solid fuels. These alkali vapors may react withthe sulfur-laden sorbent particles forming alkali sulfides on thesorbent's surfaces and preventing further continued effective sorption.Alkali-gettering materials such as diatomaceous earth, emathlite silica,bauxite, vermiculite, hectorite, and kaolin, may be injected throughthird introduction means 27 to capture such alkali vapors. Injection ofalkali-gettering materials also further enhances the dynamic filterefficiency of the bimodal agglomeration process due to the furtherincrease in the mass loading of the larger agglomeration foci in theflue gas which removes fines particulate from the gas stream. Further,in the embodiment shown in FIG. 2, additional air may be injectedthrough inlet 33 for further increasing the rate of collisions betweenthe particles during agglomeration.

Pulse combustor means 10 is illustrated in FIG. 2 in communication witha particulate collection/removal means 20 having the basic arrangementdescribed above with respect to FIG. 1. Collection/removal means 20includes a cyclone 72, a duct port 74 and, in addition, may include asolids holding tank 76 for further holding particulates removed from thegas stream. Cyclone 72 has an outlet port 73 through which the cleanedgas stream may pass to a turbine (not shown) or other gas operateddevice. Cyclone 72 may be heated as previously described, may beambiently operated cyclone, or water-cooled, whichever is preferred. Ina heated arrangement, a conventional heating unit 80 may provide theheat to cyclone 72 and other parts of the apparatus.

While the present invention is illustrated as relating to a system foroperation of a gas powered device, the same concept can be applied toany system that requires or is beneficiated by a clean gas stream, orcleaning of the gas stream prior to its introduction to the atmosphere.Consequently, other apparatus such as boilers, heaters or the like couldbe located between the pulse combustor system and the particulateremoval system.

FIG. 3 illustrates an embodiment of the present apparatus employed as anemissions control subsystem, being added to an existing gas streamconduit such as from any combustor, by way of example. Like numeralsrepresent like members as shown in FIGS. 1, 2 and 3. The exemplarycombustor (not shown) exhausts a gas stream through conduit 100. A pulsecombustor generally 10 is positioned within conduit 100, or otherwise incommunication therewith so long as the acoustic field acts on the gasstream within conduit 100. The particulate-containing gas stream flowingfrom the combustion system (not shown) along conduit 100 forms acombined particulate stream with the hot combustion products streamproduced by the pulse combustor means 10. Pulse combustor means 10 mayinclude the previously described elements, but at least the basic valvemeans, combustion chamber and resonance tube.

As previously described, a second particulate is introduced into the hotcombustion product stream from pulse combustor 10 through secondparticulate introduction means 15. The second particulate is preferablyintroduced near the juncture of combustion chamber 14 and resonance tube16, which allows for the high temperature effect to act on the secondparticulates. In instances when adequate particle size distributiondifferences exist between particles in conduit 100 and in the productstream from pulse combustor 10, further particulate addition may not benecessary.

The combined particulate gas stream formed by the exhaustion productswithin conduit 100 and the combustion products from pulse combustor unit10 lead to agglomerate formation in conduit 100. The acoustic fieldproduced by the pulse combustor 10 enhances this agglomeration such thatthe agglomerates may then be routed to a conventional collection/removalmeans (not shown). After particulate removal, the flue gas may power aturbine or other device or be released as clean effluent to theatmosphere.

An eductor 110 is located within conduit 100 generally at the locationwhere exhaust products from the existing combustor and the combustionproducts from the pulse combustor merge into a combined stream. Eductor110 may be a conventional device that allows for mixing of gases, andshould be placed in a region of high gas flow acceleration andsubsequent deceleration to further enhance particle disentrainment andmass transfer. The eductor then promotes particulate agglomeration dueto the difference in the degree of disentrainment of the largerparticulate versus the smaller particulates (normally easily entrainablesolid ash particles). Incorporation of an eductor 110 also facilitatesair staging for enhanced NO_(x) emissions control, superior acousticcoupling between the resonance chamber 16 and the area of the conduit100 where acoustic agglomeration occurs and provides for good mixingbetween the solid and gas phases in the stream.

FIG. 4 schematically illustrates pulse combusted particulate removalapparatus used as an add-on subsystem as described with respect to FIG.3. Particulate removal apparatus of the present invention generally 200is shown in line between an existing combustion system 210 and a gasoperated device 240 and includes a pulse combustion means 220 and aparticulate collection/removal means 230. The system may be arranged asshown in FIG. 3 or otherwise. Likewise as noted above, for someoperations, the clean-up may follow the operating system such as aboiler for the principle reason of effluent cleaning.

In certain particular embodiments of the present invention, a pulsecombustion chamber design of the type shown in FIG. 5 is preferred. Thisdesign employs quadratic form generators to define an axisymmetricgeometry that would be alike to accommodate a number of design andchamber performance attributes.

Alphanumeric legends on the pulse combustor illustrated in FIG. 5correspond to following dimensions which relate to a slagging combustordesign (as described hereinafter) having a heat output of 7.5 MMBtu/hrand may be used for determining other pulse combustor designs. Inletport 100 has a diameter of 5.69 inches and exit port 101 has a diameterof 5.06 inches. The lengths of the different sections of the combustionchamber are as follows: L₁ is 16.17 inches; L₂ is 4.15 inches, L₃ is4.31 inches, L₄ is 3.40 inches with a combined length of the combustionchamber from inlet port 100 to exit port 101 of 28.03 inches. The angleα is 40°, length R1 is 25.15 inches, length R2 is 6.46 inches, length R3is 4.31 inches and length R4 is 3.40 inches.

It has been found that certain ranges are preferred for the operation ofthe presently claimed pulse combusted particulate removal apparatus.Preferably, sound pressure levels attendant to the pressure wavegenerated by the pulse combustor unit although lower sometimes,preferably are at least 160 dB at atmospheric pressure: 180 dB at 10atmospheres, and 200 dB at 20 atmospheres. As previously described, thepreferred frequency range for the oscillations of the acoustic wavegenerated by the pulse combustor should be in the range of from about 20to 1500 Hz, with a range of from about 50 to about 250 Hz beingpreferred. The preferred size distribution difference betweenparticulate entrained in the gas stream which is to be removed andsecond introduced particulate should preferably be such that size of theintroduced second particulate is greater than that of the particulateinitially entrained in the gas stream. Smaller particulate preferablyshould not exceed 50 percent by weight of the total particle weight.Preferably, solids mass loading should be not less than 10 g/m³. Thepreferred mean residence time of the particulate in the resonance tube16 is from about 2 to about 5 seconds. The preferred gas combustiontemperature of the system should be maintained at a temperature lessthan the temperature at which the particulates being agglomerated beginto slag. This lower temperature prevents the formation of moltenmaterials (slag) and thus insures continued entrainment of theparticulate in the gas stream prior to and after agglomeration thereof.Preferably, the gas temperature of the system should be maintained atleast 200° F. lower than the softening or initial deformationtemperatures of the particular solids. Additionally, the pulse combustorshould preferably release heat in a range of from about 1 to about 6MMBtu/hr.

Although preferred embodiments of the invention have been describedusing specific terms, devices, concentrations, and methods, suchdescription is for illustrative purposes only. The words used are wordsof description rather than of limitation. It is to be understood thatchanges and variations may be made without departing from the spirit orthe scope of the following claims.

What is claimed is:
 1. Improved apparatus for removal of particulatesentrained in a gas stream comprising:means for receiving a gas streamhaving particulates therein for passage of said gas stream therethrough;pulse combustor means in communication with said gas stream receivingmeans for producing a pulsating stream of hot combustion products and anacoustic wave having a frequency within the range of from about 20 toabout 1500 Hz which acts on said gas stream, said pulse combustor meansfurther including means for introducing a second particulate into saidgas stream so that acoustically-enhanced bimodal agglomeration ofparticulate materials entrained in said gas stream occurs permittingenhanced removal of said agglomerated particulate materials. 2.Apparatus as defined in claim 1 wherein said pulse combustor meanscomprises a combustion chamber, fuel valve means in communication withsaid combustion chamber so that fuel mixtures may be admitted to saidcombustion chamber, and a resonance tube in communication with saidcombustion chamber.
 3. Apparatus as defined in claim 1 wherein saidpulse combustor means is located within said gas stream receiving means.4. Apparatus as defined in claim 1 wherein said second particulateintroduction means is located such that said second particulate will beintroduced into said apparatus at a point so that said acoustic wave mayact on said second particulate to enhance agglomeration between saidsecond particulate and said particulate entrained in said gas stream. 5.Apparatus as defined in claim 2 wherein said second particulateintroduction means is located near the junction of said combustionchamber and said resonance tube.
 6. Apparatus as defined in claim 1further comprising means for removing said agglomerates from said gasstream.
 7. Apparatus as defined in claim 6 wherein said removal means isa cyclone.
 8. Apparatus as defined in claim 7 wherein said cyclone isheated.
 9. Apparatus as defined in claim 1 further comprising means forintroducing a third particulate into said gas stream.
 10. Improvedapparatus for removal of particulates entrained in a gas streamcomprising:means for receiving a gas stream for passage of said gasstream therethrough; pulse combustor means in communication with saidgas stream receiving means for producing a pulsating stream of hotcombustion products having a first particulate entrained therein and anacoustic wave having a frequency within the range of from about 20 toabout 1500 Hz which acts on said gas stream, said pulse combustor meanscomprising a combustion chamber, fuel valve means in communication withsaid combustion chamber so that fuel mixtures may be admitted to saidcombustion chamber, and a resonance tube in communication with saidcombustion chamber, said resonance tube extending into said gas streamreceiving means; and means for introducing a second particulate intosaid gas stream so that enhanced bimodal agglomeration of saidparticulates occurs to produce agglomerates, said second particulateintroduction means located generally at the junction of said resonancetube and said combustion chamber.
 11. Improved process for removal ofparticulates from a gaseous stream comprising the steps ofbringing a gasstream having particulates entrained therein under the influence of apulse combusted acoustic pressure wave at a frequency in a range of fromabout 20 to about 1500 Hz; and introducing into said gas stream a secondparticulate having a size distribution different than particulatesentrained in said gas stream, said second particulate being introducedat a location under the influence of said acoustic pressure wave toeffectuate acoustic enhanced bimodal particulate agglomeration.
 12. Aprocess as defined in claim 11 wherein said pulse combusted acousticpressure wave accompanies a hot combustion product stream which containsparticulate matter to be agglomerated and is merged with a further gasstream in which other particulates are entrained.
 13. A process asdefined in claim 11 further comprising the step of removing saidagglomerates to effectuate enhanced cleaning of said gaseous stream. 14.A process as defined in claim 12 further comprising the step of removingsaid agglomerates to effectuate enhanced cleaning of said gaseousstream.
 15. A process as defined in claim 11 wherein a particulatefuel-air mixture is pulse combusted.
 16. A process as defined in claim11 wherein said second particulate has a size larger than the size ofsaid particulate entrained in said gas stream.
 17. A process as definedin claim 12 wherein said second particulate is introduced into said hotcombustion product stream so that the weight amount of the larger ofsaid particulates exceeds the weight amount of the smaller of saidparticulates.
 18. A process as defined in claim 12 wherein said secondparticulate introduced into said hot combustion product stream is asorbent for capture of contaminants.
 19. A process as defined in claim18 wherein said sorbent is selected from the group consisting oflimestone, dolomite, lime, and hydrated lime.
 20. A process as definedin claim 18 wherein said contaminants captured by said sorbent includesulfur derivatives.
 21. A process as defined in claim 11 wherein saidparticulate in said gas stream is fly ash produced by the combustion ofcoal.
 22. A process as defined in claim 11 wherein said acousticpressure wave has a frequency within a range of from about 50 to about250 Hz.
 23. A process as defined in claim 12 wherein the heat producedby said pulse combustion has a temperature less than the temperature atwhich the said particulates in the streams begin to form slag.
 24. Aprocess as defined in claim 12 wherein said gas stream containing otherparticulates which merges with said hot combustion product stream is theexhaust from a different combustion system.
 25. A process as defined inclaim 11 further comprising the step of introducing a third particulateinto said gas stream in which particulate is entrained.
 26. A process asdefined in claim 25 wherein said third particulate is analkali-gettering agent.
 27. A process as defined in claim 26 whereinsaid alkali-gettering agent is selected from the group consisting ofdiatomaceous earth, emathlite, silica, bauxite, vermiculite, hectorite,and kaolin.
 28. A process as defined in claim 13 wherein said gas streamremains hot during removal of said agglomerates therefrom.
 29. A processas defined in claim 13 wherein said gas stream which has been cleaned isprovided to a device employing a gas stream for operation thereof.
 30. Aprocess as defined in claim 29 wherein said device employing a gasstream for operation thereof is a turbine.
 31. A process as defined inclaim 11 wherein heat produced during said process is maintained in arange of from 1 MMBtu/hr to about 6 MMBtu/hr.
 32. A process as definedin claim 11 wherein the gas stream containing the particulate is a hotpulse combustion product stream that is accompanied by said pressurewave.
 33. Improved process for removal of particulates from a gaseousstream comprising the steps of:pulse combusting a fuel to produce a hotcombustion product stream having particulate material entrained thereinand an acoustic pressure wave having a frequency within the range offrom about 20 to about 1500 Hz; introducing into said hot combustionproduct stream a second particulate having a size larger than the sizeof said particulate entrained in said hot combustion product stream toeffectuate acoustically-enhanced bimodal agglomeration of saidparticulates; and removing said particulate agglomerates from said hotcombustion product stream to produce a cleaned gas stream.
 34. A processas defined in claim 33 wherein said process further comprises the stepof providing said clean gas for operation of downstream equipment.
 35. Aprocess as defined in claim 33 wherein said second particulate is asorbent for sulfur, resulting in sulfur removal from said gas stream.36. Improved process for removal of particulates from a gaseous streamcomprising the steps of:pulse combusting a fuel to produce a hotcombustion product stream and an acoustic pressure wave at a frequencyin a range of from about 20 to about 1500 Hz; merging said productstream and pressure wave with an independent gas stream havingparticulates entrained therein; bringing a second particulate materialunder the influence of said acoustic pressure wave, said secondparticulate material having a size distribution different than saidparticulate material entrained in said gas stream so that acousticallyenhanced bimodal particulate agglomeration occurs; removal of saidparticulate agglomerates from said merged streams; and thereafterproviding said merged gas streams for operation of downstream equipment.37. A process as defined in claim 36 wherein said second particulate islarger than said particulate entrained in said gas stream.
 38. Improvedprocess for removal of particulates from a gaseous stream comprising thesteps of:pulse combusting a fuel to produce a hot combustion productstream having a particulate entrained therein and an acoustic pressurewave having a frequency within the range of from about 20 to about 1500Hz; and introducing into said hot combustion product stream a secondparticulate having a size different than the size of said particulateentrained in said hot combustion product stream foracoustically-enhanced bimodal particulate agglomeration, said secondparticulate being a sorbent for effectuating sorption of contaminantspresent in said gaseous stream.
 39. A process as defined in claim 38wherein said gaseous stream is said hot combustion product streamproduced by said pulse combustion.